Various small-size image projection devices have been proposed which two-dimensionally deflect and scan a light from a light source and project an image (for example, Patent Documents 1, 2, and Non-Patent Document 1). As means (optical scanning means) for two-dimensionally deflecting a light, a galvanometer mirror or a so-called MEMS (Micro Electro Mechanical Systems) mirror is used.
Such a small-size image projection device is characterized in that the device itself is small. Therefore, a projection can be performed with the device itself being held in a hand. However, holding the device in a hand for a long time is not preferable because it causes fatigue. Therefore, it is conceivable that the device is installed on a desk or a floor to perform a projection. In such a case, however, if the projection is performed simply toward the front side, a part of a projected image is projected onto the desk or the floor, thus causing a void (vignetting) of the projected image. Thus, a projection method (so-called oblique projection method) is adopted in which a light is incident obliquely with respect to the center of a screen (projection surface) to which an image is projected. However, in a case of a deflection/scanning device having no optical system (projection optical system) which projects a deflected light to a projection surface, a defect (trapezoidal distortion) is caused in which an image (projected image) on the projection surface is distorted into a trapezoidal shape. Along with the distortion, non-uniformity of a scanning angle occurs between an upper portion and a lower portion of the screen, and thus non-uniformity of the resolution on the screen also occurs. Therefore, a high-quality image cannot be projected.
In such an image projection device, to project a moving image to a large screen, it is necessary to drive a galvanometer mirror or a MEMS mirror at a high speed and with a large amplitude (mechanical deflection angle), and for example, a driving method using a resonance (resonance driving) is adopted. However, it is known that in a case where the resonance driving is adopted, a scanning speed largely drops at both ends of the amplitude which mean the maximum value and the minimum value of the amplitude, in other words, near the vicinity of an image. Due to this phenomenon, a circumferential portion of the screen is relatively brighter than the center of the screen, and in a case where a modulation rate of a light source is constant, a difference of the resolution occurs between the center of the screen and the circumferential portion of the screen. They are known as typical problems.
Against these problems, for example, it is conceivable to realize a correction of an image distortion and a uniform speed of a light flux radiated to a projection surface such as the screen, by using a projection optical system. For example, there has been proposed an optical scanning device which performs correction of a uniform speed of scanning and an image distortion by giving so-called f-arcsine characteristics to a projection optical system and using a projection optical system including a combination of two mirrors and a single refractive lens or a combination of three mirrors (for example, Patent Document 1). There have been also proposed a projector and an image projection system in which distortion detection means is provided to control the rotation of a deflecting mirror (for example, Patent Document 2). Moreover, there has been proposed a technique of performing correction of a uniform speed of scanning and an image distortion by utilizing an image processing (for example, Non-Patent Document 1).